Foaming of thermoplastics in the majority of applications is now accomplished with exothermic foaming agents, usually azo compounds, which when admixed with the desired thermoplastic polymer in an extruder or molding machine are heated above their decomposition temperatures, and produces nitrogen which forms the cell structure in the molded or extruded part. However, in the compounding and producing of concentrates of these exothermic foaming materials the chemical reaction of the foaming agents is not suppressed in any of the current processes employed. Approaches vary, one is compounding the exothermic foaming agents in a very low melting material so that when mixing the foaming agent with the material the decomposition temperature is not reached; however, the low melting material may not be fully compatible with the desired thermoplastic polymer used to produce a foamed part or product. Another approach is to attempt to produce the concentrate at maximum flow rates or minimum residence times in the extruder. However, the conditions heretofore used are often contrary to controlling the decomposition of the foaming agent and the foaming agent reacts, releasing the gas and the pellets containing the foaming agent are already foamed or have lost the gas. These pellets are larger than the desired pellet size and have an outer surface which shows the loss of gas. Therefore, when such pellets are added to the thermoplastic resin desired to be foamed there is a non-uniform amount of unreacted chemical foaming compounds in these pellets to foam the thermoplastic and the foamed product is neither consistent nor uniform causing substantial off-specification and rejected product.
Attempts to produce endothermic chemical foaming concentrates in the high melting thermoplastic resins has not been attempted since the temperatures in an extruder to compound exceed the reaction temperature of such foaming agents. Attempts may have been made to foam thermoplastic polymers with endothermic chemical foaming agents by mixing them in waxy carriers; however, the ability to mix the foaming agent uniformly with a high melting polymer in a single pass through an extruder and produce a consistent foamed product was impossible due to melting temperature parameters between the waxy material, the foaming agents and the high melting thermoplastic resins such as polypropylene, polystyrene or polyethylene to be foamed.
U.S. Pat. Nos. 5,037,580; 5,045,570; 5,009,809 and 5,009,810 disclose chemical foaming agents specifically a polycarboxylic acid and an inorganic carbonate which are surface treated before compounding with materials such as mono-glycerides, stearic acid, silane coupling agents, fatty acids, titanates and mixtures thereof. In each patent, the admonition is given regarding compounding that: "The extruder must, however, be operated below about 149.degree. C. in order to keep the endothermic blowing agent from foaming prematurely." Hence, it is clear from the patents themselves that these foaming agents were not to be compounded in high melting resins that had melting or compounding temperatures above 149.degree. C. The patents also disclose as the material to surface treat the blowing agent a component selected from the group monoglycerides, stearic acid, silane coupling agents and mixtures thereof. U.S. Pat. No. 5,045,570 additionally includes the di-glycerides, fatty acids, fatty acid salts, hydrotitinates and zirconates as materials to surface treat the blowing agents.